1. Field of the Invention
This invention relates to electrical and electronic appliances and other equipment which accept and produce audible output in order to communicate information to users. The disclosure applies to such appliances and equipment, whether the sound produced is speech, music, alarm, signal, or any other information that the ear can hear and the mind appreciate. In particular, the invention relates to control of the loudness of: (a) received voice, and (b) the incoming call signal, in telephonic instruments.
2. Description of the Prior Art
Appliances and equipment which produce audible output often have means (called volume controls) by which a user can adjust the level of sound intensity (MVC, for short). Where audible output level is likely to fluctuate widely over time due to a varying input signal, or as a result of variations within the appliance itself (including power mains effects), an automatic sound level or volume control (AVC, for short) is conventionally used. AVC, through the mechanism of inverse feedback, results in a reduction in the range of output signal fluctuation relative to a wider range of input signal fluctuation or other variations due to changes in the appliance itself (including power mains effects).
The ability of a listener to hear and appreciate audible sound produced by an appliance or other equipment is dependent on the intensity of such sound in comparison to that of background audible noise present in the listening area. A familiar problem, in many situations where appliances and other equipment that produce audible sound are used, is interference from ambient audible noise that varies with time. An MVC setting that is suitable under quiet conditions may produce too low a level of output when background noise increases. Conversely, the MVC setting used when loud background noise is present may cause a disturbingly loud output when quiet backgound conditions exist. Two familiar cases of this shortcoming in existing telephonic instruments are noted here:
The first case involves a telephone user attempting to carry on a conversation in a room in which the background noise level varies. Most electronic phones have line length compensation circuits (AVC) that maintain average signal level constant independent of the subscriber's distance to the central office; these circuits do not solve the problem of fluctuating, audible background noise. Some telephones provide MVC. If the phone is equipped with MVC, then a user attempting to hear a message in the presence of varying background noise, may find it necessary to frequently divert attention from the conversation in order to reset the control. PA1 The second case involves incoming telephone call signals which may not be heard in a noisy room. Older, electromechanical phones are equipped with a "clapper" bell that often have a mechanical lever to set the loudness level of the incoming call signal. Electronic phones employ an audio transducer, which may also have an adjustment for loudness. When phones with a loudness adjustment for the incoming call signal are used in areas where there is varying background noise level, a setting that is suitable under quiet conditions, may be inadequate when loud noise is present. Conversely, the adjustment, that is suitable when loud noise is present, will, in a quiet room or at night, be unnecessarily disturbing.
For those wishing information about prior art in these areas, reference is made to one of the standard textbooks or handbooks on the subject. Recommended are J. Atkinson TELEPHONY (London vols. 1 and 2, 1968, Pitman and Sons Ltd.); "The Electronic Telephone", Peter P. Luff, SCIENTIFIC AMERICAN, March 1978, pages 58-64; AND ELECTRONIC CIRCUITS, D. L. Schilling and C. Belove, (McGraw-Hill, New York, 1979).